1. Field of the Invention
The invention relates broadly to sensing devices for detecting ferrite and non-ferrite objects behind wall linings. Such a device is commonly called a metal and stud detector or finder. More particularly, the invention relates to a device and method for detecting ferrite and non-ferrite objects in proximity of an inductively coupled sensor.
2. Background Information
The use of inductively coupled sensors having a transmitter (or excitation) coil and a receiver coil for detecting the presence of metal objects is well known. The transmitter coil is excited with a periodically varying excitation signal which produces and alternating magnetic field. The magnetic field induces a sensor signal in the receiver coil. The presence of ferrite objects in proximity to the coils affects the inductive coupling between the coils. In particular, metal objects within the proximity cause a phase shift between the excitation signal and the induced sensor signal, which can be used to indicate the presence of the metal object. Hitherto metal detectors employing this type of sensor have suffered drawbacks including calibration stability and high processing demands on the detection circuit. To examine the amplitude and phase properties of received signals known devices typically recorded the whole waveform and then calculate changes at every point on the waveform. This creates a considerable processing overhead that requires use of a powerful and expensive DSP type processor.
In order to overcome problems with calibration stability known devices must be calibrated by a user calibration or automatic calibration function immediately prior to each use. This makes manufacture and use of the device more complicated and introduces a calibration delay at the beginning of each use of the device.
It is an objection of the present invention to provide a metal detector and a method of detecting the presence of a metal object using an inductive type sensor that overcomes or at least ameliorates some or all of the above problems.